A unit of China's People's Liberation Army had stopped computer intrusions in February after the administration of President Barack Obama said it was responsible for numerous thefts of intellectual property and government documents in the past five years, officials said.
There have been new hacking attacks, The New York Times reported Sunday.
Mandiant, a private security firm that helps companies defend themselves from hackers, said the attacks resumed but would not identify those affected, citing agreements with clients.
It did say the new attacks had targeted some of the same companies hacked previously.
Crowdstrike, another cybersecurity firm tracking the Chinese hackers, agreed with Mandiant's findings. Crowdstrike's director of intelligence said it was 'business as usual" for the Chinese.
Obama administration officials say they weren't surprised by the resumption of attacks.
"This is something we are going to have to come back at time and again with the Chinese leadership," a senior official said Friday. The Chinese "have to be convinced there is a real cost to this kind of activity."
Caitlin Hayden, spokeswoman for the National Security Council, said the administration hoped to win "longer-term changes in China's behavior, including by working together to establish norms against the theft of trade secrets and confidential business information."
The Chinese foreign ministry has denied any involvement in the attacks.
The People's Daily in China has called the United States "the real 'hacking empire."